1. Field of the Invention
This invention relates to a fluidized catalytic cracking process in the presence of a conventional catalyst and a zeolite which has been given a thermal treatment prior to use.
2. Description of the Prior Art
Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking and such disclosure appears in one or more of the above-identified U.S. patents.
It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is included with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202.
Improved results in catalytic cracking with respect to both octane number and overall yield were achieved in U.S. Pat. No. 3,758,403. In said patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 zeolite to large pore size crystalline zeolite was in the range of 1:10 to 3:1.
The use of ZSM-5 in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 in amounts up to and about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 to large pore size crystalline zeolite in the range of 1:10 to 3:1.
The addition of very small amounts of pure, finely divided shape selective catalyst to a conventional FCC catalyst, was taught in U.S. Pat. No. 4,309,280, the entire contents of which are incorporated herein by reference. This patent taught the advantage of using, as the powdered additive catalyst, a ZSM-5 zeolite with very high silica-alumina ratios. Use of a 1500 to 1 SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio ZSM-5 catalyst in conjunction with a fluid cracking process was disclosed in Example 8 and Example 13. Use of ZSM-5 with an even higher ratio was disclosed in Example 9, which added ZSM-5 containing only 15 ppm Al.sub.2 O.sub.3. The patentees commented that the preferred additives, shape selective zeolites such as ZSM-5, were very active even with high silica to alumina mole ratios. This activity was considered surprising, since catalytic activity of zeolites is generally attributed to cations associated with framework aluminum atoms.
U.S. Pat. No. 4,309,279, the entire contents of which are incorporated herein by reference, disclosed the addition of very small amounts of a special class of zeolites characterized by a silica to alumina mole ratio greater than 12 and a constraint index of about 1 to 12, to conventional cracking catalyst. The patentees included a teaching, but no examples, to addition of shape selective zeolites, e.g., ZSM-5, with very high silica to alumina mole ratios, up to 30,000 and greater.
In U.S. Pat. No. 4,340,465, the entire contents of which are incorporated herein by reference, the patentees taught use of ZSM-5 with very high silica to alumina mole ratios for catalytic cracking. The examples in this patent show that as the silica to alumina mole ratio increased, the activity of the catalyst, as measured by the weight percent conversion, decreased. In going from a sieve containing 2.3 weight percent Al, to 0.45 to 0.04 wt %, the weight percent conversion declined from 34 to 12 to 7, respectively. This indicated a significant loss in cracking activity when using ZSM-5 with a relatively low aluminum content.
Some work has been done on removal of alumina from relatively small pore zeolites such as mordenite. In U.K. patent specification No. 1,151,653, the patentees disclosed that the hydrogen form of a zeolite was preferred for many hydrocarbon conversion processes, and taught a combination treatment of the zeolite with acid and an ammonium compound to achieve the desired hydrogen form. The treatment included boiling with acid, which would extract some aluminum.
U.K. patent specification No. 1,261,616 taught a method of making acid extracted mordenite.
Some work has also been reported on preparation of aluminum deficient faujasites in "Chemistry of Crystalline Aluminosilicates", G. T. Kerr, the Journal of Physical Chemistry, Vol. 72, 1968, pages 2594-2596 and in U.S. Pat. No. 3,442,795. The entire contents of these references are incorporated herein by reference. Aluminum was removed directly from sodium zeolite Y, using ethylenediaminetetraacetic acid, EDTA. This reference taught that as most of the Al was removed from the NaY the crystallinity of the material changed, and indeed was lost when less than 20 percent of the original aluminum framework content remained. This reference reported increased sorptive capacity, based on the number of grams of SiO.sub.2 in the samples, up to about 70 percent aluminum removal, after which sorptive capacity decreased.
The art has recognized the desirability of adding ZSM-5 material, preferably with a low aluminum content in the ZSM-5 framework, to conventional FCC catalyst. Most of the art's efforts at achieving low aluminum content have been directed towards either producing a material with a very low aluminum content, by, e.g., making ZSM-5 material with a silica sol that contained only a very small amount of aluminum. Another way of achieving this low aluminum content is to use conventional aluminum extraction methods, such as treating the ZSM-5 with a strong mineral acid or a chelating agent such as ethylenediaminetetraacetic acid.
It is also known that the alpha activity of ZSM-5 catalyst can be modified by thermal treatment. In U.S. Pat. No. 4,016,218, the entire contents of which are incorporated herein by reference, a process for the alkylation of aromatic hydrocarbons using thermally treated ZSM-5 catalyst is disclosed. Thermal treatment is used to reduce the alpha activity to less than 250, and preferably less than about 200 but greater than 10.
We have discovered that it is possible to achieve the benefits of adding low alumina shape selective material, e.g., ZSM-5, to a conventional FCC process, without the expense and inconvenience of making a zeolite with an inherently low alumina content, or acid extracting a conventional zeolite.
We can take ZSM-5 with, e.g., 40-90 silica to alumina mole ratio, and convert this into material that acts in the FCC process as if it had a relatively low aluminum content equivalent to a 500:1 to 1500:1 silica to alumina ratio.
We learned that a thermal treatment of conventional ZSM-5 could significantly alter the characteristics of this shape selective catalyst so that it responded in the FCC process, as a shape selective catalyst that had been made by a special manufacturing techniques, or acid extracted, to contain a relatively low aluminum content.
The process that we use to alter the catalytic activity of the shape selective material is a simple one--steaming or other thermal treatment. The simplicity and efficacy of steaming to achieve these results is surprising because the art has been steaming FCC catalysts for years without recognizing the benefits that could be obtained by controlled steaming of these shape selective materials.